1. Field of the Invention
The present invention relates to a method and device for measuring blood flow rate in a blood access. Blood is taken out from the body of a mammal to an extracorporeal blood circuit through a blood access, via needles or a catheter.
2. Description of the Related Art
There are several types of treatments in which blood is taken out in an extracorporeal blood circuit. Such treatments involve, for example, hemodialysis, hemofiltration, hemodiafiltration, plasmapheresis, blood component separation, blood oxygenation, etc. Normally, blood is removed from a blood vessel at an access site and returned to the same blood vessel or at another location in the body.
In hemodialysis and similar treatments, an access site is commonly surgically created in the nature of a fistula. Blood needles are inserted in the area of the fistula. Blood is taken out from the fistula via an arterial needle and blood is returned to the fistula via a venous needle.
A common method of generating a permanent access site having capability of providing a high blood flow and being operative during several years and even tens of years, is the provision of an arterio-venous fistula. It is produced by operatively connecting the radial artery to the cephalic vein at the level of the forearm. The venous limb of the fistula thickens during the course of several months, permitting repeated insertion of dialysis needles.
An alternative to the arterio-venous fistula is the arterio-venous graft, in which a connection is generated from, for example, the radial artery at the wrist to the basilic vein. The connection is made with a tube graft made from autogenous saphenous vein or from polytetrafluorethylene (PTFE, Teflon). The needles are inserted in the graft.
A third method for blood access is to use a silicon, dual-lumen catheter surgically implanted into one of the large veins.
Further methods find use in specific situations, like a no-needle arterio-venous graft consisting of a T-tube linked to a standard PTFE graft. The T-tube is implanted in the skin. Vascular access is obtained either by unscrewing a plastic plug or by puncturing a septum of said T-tube with a needle. Other methods are also known.
During hemodialysis, it is desirable to obtain a constant blood flow rate of 150-500 ml/min or even higher, and the access site must be prepared for delivering such flow rates. The blood flow in an AV fistula is often 800 ml/min or larger, permitting delivery of a blood flow rate in the desired range.
In the absence of a sufficient forward blood flow, the extracorporeal circuit blood pump will take up some of the already treated blood entering the fistula via the venous needle, so called access or fistula recirculation, leading to poor treatment results.
The most common cause of poor flow with AV fistulas is partial obstruction of the venous limb due to fibrosis secondary to multiple venipunctures. Moreover, stenosis causes a reduction of access flow.
When there is a problem with access flow, it has been found that access flow rate often exhibit a long plateau time period with reduced but sufficient access flow, followed by a short period of a few weeks with markedly reduced access flow leading to recirculation and ultimately access failure. By constantly monitoring the evolution of the access flow during consecutive treatment sessions, it is possible to detect imminent access flow problems.
Several methods have been suggested for monitoring recirculation and access flow. Many of these methods involve injection of a marker substance in blood, and the resultant recirculation is detected. The methods normally involve measurement of a property in the extracorporeal blood circuit. Examples of such methods can be found in U.S. Pat. No. 5,685,989, U.S. Pat. No. 5,595,182, U.S. Pat. No. 5,453,576, U.S. Pat. No. 5,510,716, U.S. Pat. No. 5,510,717, U.S. Pat. No. 5,312,550, etc.
Such methods have the disadvantage that they cannot detect when the access flow has decreased to such an extent that recirculation is at risk, but only when recirculation prevails. Moreover, it is a drawback that injection of a substance is necessary.
A noninvasive technique that allows imaging of flow through AV grafts is color Doppler ultrasound. However, this technique requires expensive equipment.
The measurement of access flow rate necessitates the reversal of the flows in the extracorporeal circuit. A valve for such reversal is shown in i.a. U.S. Pat. No. 5,605,630 and U.S. Pat. No. 5,894,011. However, these valve constructions comprise dead ends in which blood may stand still for a long time and coagulate, which is a drawback.